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UNIVERSITY    OF    CALIFORNIA 

College  of  agriculture 

BERKELEY,   CALIFORNIA 


Agricultural  ExperimentStation 

E.  J.  WlCKSON,  Acting  Director 


CIRCULAR  No.  30 

(June,  1907.) 


WHITE   FLY  IN   CALIFORNIA 


By  C.  W.  WOODWORTH. 


The  white  fly  has  at  last  become  established  in  California.  The 
most  destructive  insect  in  the  citrus  groves  of  Florida  is  this  species 
of  white  fly,  the  Aleurodcs  citri.  It  is  considered  by  the  growers  there 
as  much  more  injurious  than  any  of  the  scale  insects  attacking  these 
fruits.     Our  trees  were  supposed  to  be  free  from  the  pest,  but  now 


Figure  1. — The  white  fly  on  a  leaf.     Natural  size. 

a  well  established  colony  has  made  its  appearance  at  Marysville  at  a 
center  of  travel  by  both  steam  and  electric  railways  where  the  condi- 
tions seem  particularly  favorable  to  its  spread  over  the  State. 

The  seriousness  of  these  facts  to  the  citrus  industry  of  this  State 
renders  it  imperative  that  full  publicity  be  given  them  in  order  that 


the  proper  steps  may  be  taken  to  meet  this  emergency.  We  are  not 
sure  that  the  insect  is  confined  to  this  one  locality.  Every  one  who 
has  trees  should  carefully  inspect  them  and  submit  all  suspicious  speci- 
mens to  some  one  capable  of  accurately'  identifying  them.  The  Ex- 
periment Station  will  be  glad  to  examine  and  report  on  any  that  may 
be  submitted  for  this  purpose.  It  may  be  entirely  possible  to  eradicate 
an  insect  before  it  has  become  well  established,  but  it  will  if  allowed 
to  increase  become  too  widely  distributed  to  effectively  handle. 

Many  have  been  very  apprehensive  of  the  danger  of  introducing 
this  insect  from  Florida  and  have  been  instrumental  in  having  most 
stringent  laws  enacted  preventing  the  importation  of  anything  from 
that  State  liable  to  be  infested,  while  others  have  maintained  that  the 
insect  could  not  live  in  this  climate  at  all.  In  support  of  this  idea  it 
has  been  claimed  that  the  insect  must  have  been  introduced  into  the 
State  many  times  on  nursery  stock  before  these  laws  went  into  effect 
and  since  that  time  on  plants  that  escaped  the  attention  of  the  inspec- 
tors. Many  other  species  of  white  flies  are  able  to  live  in  California, 
however,  including  at  least  one  injurious  species.  We  do  not  have  to 
speculate  any  longer  on  this  matter  since  the  insect  in  question  is  now 
not  only  existing  here  but  thriving.  Moreover,  the  insect  is  not  ap- 
pearing in  the  most  humid  parts  of  the  State,  but  in  the  interior  where 
the  black  scale  is  unable  to  thrive,  that  is,  in  a  climate  comparable  to 
Riverside. 

Though  we  now  know  that  the  white  fly  does  live  and  thrive  in  our 
climate  in  the  city  of  Marysville,  there  are  those  who  still  believe  that 
our  orchards  are  not  menaced,  still  arguing  on  the  basis  of  the  climatic 
differences  between  California  and  Florida.  The  red  scale,  the  black 
scale,  and  the  purple  scale  are  all  Florida  insects  and  have  not  found 
our  climate  impossible.  Possibly  the  white  fly  may  prove  to  be  dif- 
ferent, but  it  does  not  seem  the  part  of  wisdom  to  permit  the  white  fly 
to  have  free  access  to  our  citrus  orchards  on  the  chance  that  it  may 
not  thrive  there. 

LIFE  HISTOKY. 

The  white  fly  must  have  been  in  California  at  least  a  full  year 
judging  by  its  distribution  and  abundance  at  Marysville,  but  no  ob- 
servations have  been  made  on  its  life  history  and  habits  except  during 
the  last  few  weeks  since  its  discovery  in  the  State.  There  is  but  little 
doubt,  however,  that  it  will  prove  to  be  much  the  same  as  in  Florida. 
The  most  elaborate  study  of  this  insect  is  that  reported  in  Bulletin  67 


of  the  Florida  Experiment  Station  by  Professor  II.  A.  Gossard.  In 
the  following'  account  of  the  insect  much  will  be  taken  from  this  pub- 
lication including  the  two  plates  illustrating  the  structure  of  the  insect. 
The  Egg.— At  the  present  time  the  insect  is  chiefly  in  the  egg  stage. 
These  are  shown  very  much  enlarged  in  figure  2,  which  was  photo- 
graphed from  a  specimen  collected  at  Marysville,  and  illustrates  how 


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Figure  2. — Eggs  of  the  white  fly. 


numerous  th£y  become.  There  were  many  thousands  of  such  eggs  on 
this  leaf.  Nearly  every  leaf  on  the  trees  in  that  part  of  town  was 
similarly  infested.  These  eggs  are  so  small  as  to  be  scarcely  visible  to 
the  eye  singly,  but  the  badly  infested  leaves  appeared  as  though  cov- 
ered with  mold.  While  rather  less  than  half  of  the  town  of  Marysville 
is  badly  infested,  still  a  careful  examination  would  show  the  presence 
of  a  few  eggs  on  nearly  every  tree  in  town. 

The  eggs  of  the  white  flies  are  all  very  much  alike  and  can  be  at 
once  distinguished  from  every  other  known  group  of  insects.  The 
shape  of  the  egg  and  of  the  stalk  at  the  end  by  which  it  is  attached  to 
the  leaf  are  sufficient  to  distinguish  them.  It  is  not,  however,  easy  to 
distinguish  between  the  eggs  of  the  different  species  of  white  flies. 


The  presence  of  an  egg  of  this  character  on  the  leaf  of  an  orange  or 
lemon  is  quite  conclusive  evidence  of  the  presence  of  the  destructive 
white  fly  of  Florida. 

According  to  Gossard,  the  egg  requires  from  three  to  twenty  days  to 
hatch,  according  to  the  weather.  In  the  hatching  process  the  egg-shell 
splits  in  the  manner  shown  in  plate  1,  figure  4. 

The  Nymph.— The  young  insect  on  hatching  very  much  resembles 
a.  young  scale  insect,  but  can  be  readily  distinguished  by  comparing  a 


Figure  3. — Exuvium  of  the  white  fly  and  a  young  nymph.' 


carefully  mounted  specimen  under  a  medium  or  high  power  of  the 
microscope  with  the  figure  shown  on  plate  2,  figure  1.  Like  the  scale 
insects,  the  young  white  fly  wanders  over  the  twigs  and  leaves  for  a 
few  hours,  finally  attaching  itself  by  inserting  its  mouthparts  into  the 
leaf.  After  thus  locating  itself  the  young  insect  soon  secretes  a  wax 
in  sufficient  quantity  to  fill  the  space  between  the  body  and  the  leaf. 
Because  of  the  small  size  and  transparency  of  the  body  and  this 
cementing  of  the  body  to  the  leaf  the  insect  is  almost  invisible.  This 
makes  it  rather  difficult  to  secure  a  good  photograph  of  the  insect. 
Figure  3  illustrates  a  partly  grown  nymph  collected  at  Marysville 
produced  from  eggs  laid  this  spring  by  flies  appearing  before  the  great 
majority  had  emerged. 


In  structure  and  habits  the  nymph  is  so  like  scale  insects  that  if 
seen  at  all  they  could  easily  pass  as  young  scales.  It  is  not  safe  indeed 
to  depend  on  the  use  of  a  hand  magnifier  for  the  identification  of  a 
young  or  partly  grown  nymph  until  one  is  thoroughly  familiar  with 
them,  and  even  then  the  use  of  a  compound  microscope  should  be 
resorted  to.  The  details  of  structure  shown  by  a  properly  mounted 
microscopical  preparation  of  the  various  stages  is  well  shown  on 
plate  2. 


Figure  4. — White  fly  exuviae  on  leaf,  much  enlarged. 


Within  the  body  of  the  nymph  during  the  last  stage  wings  are 
developed  in  both  sexes  and  the  whole  internal  structure  reorganized 
so  that  when  the  skin  is  finally  cast  off  a  slender  bodied,  delicate  fly- 
emerges.  This  is  in  striking  contrast  to  the  life  history  of  scale  insects 
where  the  male  alone  obtains  wings.  This  makes  the  problem  of  the 
control  of  this  insect  entirely  different  and  much  more  difficult  than 
that  of  scale  insects,  as  will  be  shown  in  the  discussion  of  this  subject 
below. 

The  full  grown  nymphs  have  a  very  much  thicker  skin  than  the 
younger  stages  and  this  remains  attached  to  the  leaf  after  the  molt. 
This  dried  skin  is  very  much  more  conspicuous  than  the  living  insect 


and  is  the  form  illustrated  in  figures  1,  3,  4,  5,  and  6.  On  some  trees  at 
Marysville  most  of  the  old  leaves  are  well  spotted  over  on  the  under 
side  of  these  exuviae,  as  shown  in  figure  5. 

The  Adult  Insect. — The  name  white  fly  was  given  to  the  insects  of 
this  class  on  account  of  the  appearance  of  the  adult  insect.  Its  snowy 
white  color  makes  the  insect  very  conspicuous  notwithstanding  its  ex- 
tremely small  size.  In  the  spring  they  appear  in  great  numbers  for 
two  or  three  weeks  and  then  suddenly  disappear  after  laying  their 


Figure  5. — Leaf  showing  dirty  condition  when  badly  infested. 


eggs,  only  a  very  few  belated  individuals  coming  out  after  the  others 
remaining  in  the  trees.  In  mid-summer  a  second  brood  begins  to  ap- 
pear. There  is  no  distinct  swarm  as  in  the  spring,  but  during  the 
whole  latter  half  of  the  year  the  adult  white  fly  can  be  seen  on  the 
trees  in  greater  or  less  abundance.  After  the  colder  weather  of  winter 
begins  the  adults  almost  entirely  disappear  and  are  not  seen  again 
until  the  spring  emergence.  The  life  of  any  individual  fly  is  very 
brief,  averaging,  according  to  Gossard,  between  four  days  and  a  week. 
During  this  time  the  eggs  are  laid,  and  as  many  as  seventy-five  have 
been  deposited  by  a  single  female. 


*" 


EXPLANATION  OF  PLATE  I. 

Aleyrodes  citri. 

(This  plate  is  reproduced  from  Bulletin  No.   67   of  the  Florida   Agricultural 
Experiment  Station.) 

Fig.  1. — Adult  female,  with  expanded  wings,  much  enlarged. 

Fig.  2. — Adult  female,  with  wings  folded  over  the  body  in  normal  roof-like 
position. 

Fig.  3. — Egg  and  foot-stalk  of  same,  greatly  magnified. 

Fig.  4. — Egg-shell,  showing  the  split  through  which  the  larva  emerged. 

Fig.  5. — Tip  of  male  abdomen,  showing  claspers. 

Fig.  6. — Antenna,  showing  annulated  joints. 

Fig.  7. — Fore  margin  of  front-  wing. 
Insects  sometimes  mistaken  for  A.  citri: 

Fig.  8. — Larva  of  Aleyrodes  floridensis,  greatly  magnified. 

Fig.  10. — Greatly  enlarged  section  of  waxen  fringe,  surrounding  A.   -flori- 
densis. 

Fig.  11. — Outline  of  larva  of  Lecanium  hesperidum. 


EXPLANATION  OF  PLATE  II. 

(This  plate  is  reproduced  from  Bulletin  No.   67   of  the  Florida  Agricultural 
Experiment  Station.) 

Aleyrodes  citri. 

Fig.  1. — Larva,  first  stage,  greatly  magnified. 

Fig.  2. — Larva,  first  stage,  drawn  to  same  scale  as  figures  3,  4,  5,  8,  9, 
and  10. 

Fig.  3. — Larva,  second  stage. 

Fig.  4. — Larva,  third  stage. 

Fig.  5. — Larva,  fourth  stage. 

Fig.  6. — Margin  of  advanced  larva,  greatly  enlarged. 

Fig.  7. — Vasiform  orifice  of  fourth  larval  stage,  showing  crenulated  oper- 
culum with  ligula  in  the  center. 

Fig.  8. — Pupa,  showing  embryo  and  distribution  of  orange-colored  areas. 
Waxen  tufts  extending  from  the  breathing  tubes  are  shown. 

Fig.  9. — Adult,  with  folded  wings,  emerging  from  the  pupa  case. 

Fig.  10. — Empty  pupa  case,  showing  split  through  which  the  fly  emerged. 


PLATE  I. 


+  1  ? 


PLATE  II. 


10 


THE  DETECTION  OF  THE  WHITE  FLY. 

At  the  present  time  the  discovery  of  the  presence  of  the  white  fly 
is  of  very  great  importance.  After  the  insect  has  become  so  numerous 
that  it  is  conspicuous  either  by  its  injuries  or  the  swarms  of  the  adult 
it  is  doubtful  if  eradication  is  possible,  at  least  without  very  radical 
measures  being  employed.  Much  misapprehension  can  be  avoided  by 
getting  a  correct  idea  of  the  insect  and  of  the  things  it  is  liable  to  be 
confounded  with. 


Figure  6. — Red  scale,  adult  and  young,  and  exuviae  of  white  fly. 


First  it  should  be  understood  that  the  white  fly  is  very  minute. 
There  is  no  other  insect  pest  on  citrus  trees  as  small  as  the  white  fly, 
unless  it  be  small  individuals  of  the  red  or  yellow  scale.  Figure  6 
shows  these  two  insects  on  the  same  leaf,  and  will  give  some  idea  of 
their  relative  appearance.  The  lighter  spots  are  the  exuviae  of  the 
white  fly.  There  were  a  few  young  and  eggs  of  this  insect,  but  they 
could  not  be  shown  without  magnification.  The  small  dark  spots  are 
the  young  of  the  red  scale.  The  structure  of  the  scale  is  better  shown 
in  figure  7.  The  red  or  yellow  scale  occurs  on  the  upper  side  of  the 
leaf  as  well  as  below  (see  figure  8)  and  sometimes  on  twigs  and  fruit, 


H  cT 


11 


Figure  7. — Red  scale,  much  magnified. 

while  the  white  fly  confines  itself  exclusively  to  the  leaves  and  almost 
exclusively  to  the  under  side  of  the  leaves.  No  one  will,  however, 
confound  these  two  insects  if  they  see  them  clearly,  for  they  are  so 


Figure  8. — Red  scale  on  upper  side  of  leaf. 


12 


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Figure  9. — Bed  scale  on  leaf,  much  enlarged. 

totally  different  in  appearance.  The  comparison  of  figures  4  and  9 
will  show  the  differences  that  can  be  brought  out  by  a  hand  magnifier 
between  the  exuvium  of  a  white  fly  and  the  adult  red  scale,  which 
might  be  confounded  at  first  glance  without  the  lens. 


Figure  10. — Bed  and  soft  brown  scale,  much  enlarged. 


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13 


An  insect  much  more  likely  to  be  confused  with  the  white  fly  is  the 
soft  brown  scale.  This  is  a  much  larger  insect  than  either  the  red  scale 
or  the  white  fly.  Figure  10  shows  about  the  smallest  size  of  the  adult ; 
it  is  often  more  than  twice  as  long.  One  can  very  easily  mistake,  how- 
ever, a  very  young  soft  brown  scale  for  a  partly  or  fully  grown  nymph 
of  the  white  fly.  The  soft  brown  scale  is  rarely  as  transparent  as  a 
white  fly  of  the  same  size  and  is  therefore  much  more  easily  seen.  If 
one  bends  a  leaf  just  beneath  a  young  white  fly  he  can  generally  suc- 
ceed in  causing  the  waxy  cement  that  holds  the  insect  to  the  leaf  to 


Figure  11. — The  soft  brown  scale,  much  enlarged. 


separate,  allowing  air  to  go  beneath  the  insect,  making  it  much  more 
plainly  visible,  while  no  kind  of  manipulation  produces  such  an  effect 
on  the  soft  brown  scale.  There  will  be  many  doubtful  cases,  however, 
where  the  compound  microscope  needs  to  be  brought  into  use.  When 
this  is  done  a  comparison  with  figure  11  of  plate  I  and  figures  1  to  8 
of  plate  II  will  enable  one, to  answer  the  question  with  absolute  cer- 
tainty. 

There  are  many  kinds  of  spots  to  be  found  on  orange  leaves  which 
will  be  viewed  with  suspicion,  all  of  which  can  be  readily  distinguished 
by  the  use  of  a  good  hand  lens.  We  will  figure  only  one  of  these,  the 
nature  of  which  is  not  generally  understood.     The  small,  active,  dark 


14 


colored  insects  so  common  on  orange  trees,  belonging  to  the  family 
Psocidae,  spin  a  small  white  silken  covering  for  their  eggs,  as  shown  in 
figure  12.  The  white  fly  exists  on  the  tree  only  in  four  forms:  the 
egg,  the  nymph,  the  old  finally  discarded  skin  of  the  full  grown  nymph 
called  the  exuvium,  and  the  active  flying  stage  of  the  adult  insect.  All 
of  these  are  illustrated  in  this  circular.  Any  other  kind  of  spot  or 
structure  on  the  leaf  is  certainly  something  else  than  the  white  fly. 


Figure  12. — Egg  cocoon  of  a  Psocid. 

The  means  of  distinguishing  this  species  of  white  fly  from  other 
members  of  the  same  family  is  a  rather  difficult  and  complicated 
matter,  but  can  be  done  by  a  careful  comparison  of  the  minuter  details 
shown  in  plate  II,  especially  in  figures  5  and  8. 


FOOD  PLANTS. 

The  white  fly  seems  to  prefer  the  leaves  of  orange  and  lemon,  but 
will  come  to  maturity  on  quite  a  variety  of  plants.  This  greatly  com- 
plicates the  problem  of  their  eradication  or  control.  The  following  is 
a  list  of  the  plants  upon  which  the  insect  breeds  in  Florida,  as  recorded 
by  Gossard. 


rn 


15 

Citrus,  all  species. 

China  berry  tree  (Melia  azederach) . 

Viburnum  nudum. 

Cape  jessamine  {Gardenia  florida). 

Japan  persimmon  (Diospyros  TcaJci). 

California  privet  {Ligustrum  amurense) . 

Golden  privet  {Ligustrum  sp.)  . 

Mock  orange  {Prunus  caroliniana) . 

Ficus,  various  species. 

Water  oak  {Quercus  aquatica). 

Prickly  ash  {Xanthoxylum  sp.). 

CHARACTER  OF  INJURY. 

The  injury  done  by  the  white  fly  is  of  exactly  the  same  character  - 
as  that  done  by  scale  insects,  with  the  exception  that  being  confined  to 
the  leaves  there  is  no  direct  damage  to  the  twigs  and  larger  branches 
that  makes  the  work  of  the  San  Jose  scale,  for  instance,  so  fatal  to  the 
trees  it  attacks.  Its  direct  effect  on  the  foliage  is  similar  to  that  of  the 
red  or  purple  scale  and  its  indirect  effect  like  that  of  the  black  scale. 
Doubtless  this  latter  will  be  considered  the  more  serious  since  the  ex- 
cretions of  this  insect  seem  to  cause  the  smut  to  cling  with  so  much 
tenacity  that  it  is  practically  impossible  to  remove  it. 

METHODS  OF  CONTROL. 

We  have  of  course  no  experience  in  California  in  the  control  of 
this  insect.  The  best  Florida  practice  as  recommended  by  Professor 
Gossard  is  as  follows : 

December — Spray  with  resin  soap. 

January — Spray  with  resin  soap. 

February — Spray  again  as  above  if  necessary. 

June — Spray  with  potash  whale  oil  soap. 

July — Spray  with  potash  soap  and  sulfur. 

August — Spray  with  potash  soap  and  sulfur. 

September — Spray  with  potash  soap  and  sulfur. 

Such  a  programme  of  spraying  would  seem  rather  hard  to  carry- 
out  to  our  growers,  and  possibly  we  might  find  that  two  or  three  fumi- 
gations a  year  would  be  as  effective  as  the  larger  number  of  sprayings, 
or  perhaps  one  fumigation  followed  by  several  summer  sprayings. 
Our  trees  are  mostly  low  and  small  as  compared  with  the  Florida 
groves,  so  that  fumigation  is  possible  as  it  would  not  be  in  that  State. 

None  of  the  scale  insects  can  fly,  and  therefore  distribute  them- 
selves very  slowly;  but  with  the  white  fly  in  a  community  a  cleans 
orchard  may  become  badly  infested  in  a  day,  making  summer  treat- 
ment necessary  no  matter  how  thorough  the  winter  work  has  been. 


L6 


THE  PRESENT  PROBLEM  IN  CALIFORNIA. 

As  far  as  is  known  the  white  fly  is  -still  confined  to  the  city  limits 
of  Marysville.  There  are  no  citrus  orchards  in  the  immediate  neigh- 
borhood. Possibly  therefore  it  may  not  have  reached  any  other  section. 
If  let  alone  it  certainly  will  reach  every  part  of  the  State.  The  trees 
in  the  immediate  neighborhood  of  the  railroad  tracks  are  now  in- 
fested. The  electric  cars  pass  between  infested  trees  going  to  the 
orange  orchards  of  Butte  county.  Now,  if  ever,  is  the  time  to  make 
an  effort  to  destroy  the  pest.  There  is  no  possibility  of  throwing  a 
■quarantine  around  the  infested  district  that  a  flying  insect  will  respect. 
To  delay  all  effort  towards  eradication  till  next  winter,  as  seems  now 
to  be  the  programme,  may  be  a  fatal  error.  Every  day  during  the 
latter  part  of  the  summer  thousands  if  not  hundreds  of  thousands  of 
these  minute  creatures  will  be  in  the  air  capable  of  living  long  enough 
to  be  carried,  in  a  car,  the  length  of  the  State.  A  single  one  of  these 
reaching  a  citrus  district  might  make  the  eradication  of  the  insect  an 
impossibility.  It  may  be  that  already  the  insect  has  gotten  beyond 
control,  but  the  chance  of  eradication  is  certainly  worth  the  effort. 

If  eradication  is  to  be  attempted  no  half-way  measures  like  spray- 
ing or  fumigation  are  to  be  thought  of.  There  is  too  much  opportunity 
to  miss  some  and  no  ready  means  of  discovering  the  fact.  A  campaign 
of  eradication  should  include :  first,  the  immediate  destruction  of  every 
leaf  on  all  citrus  trees  in  Marysville  and  of  every  other  plant  known 
to  furnish  sustenance  to  this  insect;  second,  a  thorough  inspection  of 
the  new  foliage  as  it  comes  out  on  the  defoliated  plants ;  third,  an  ex- 
haustive search  for  other  possible  food  plants  of  the  white  fly ;  fourth, 
redefoliation  and  persistent  reinspection  of  any  spots  where  the  fly 
may  not  have  been  annihilated ;  fifth,  careful  inspection  of  every  sup- 
posed case  of  white  fly  in  the  State;  and  finally  competent  scientific 
supervision  of  the  work. 

There  will  be  very  few  adult  flies  produced  durin.g  this  month  and 
the  early  part  of  July.  After  that  the  problem  will  be  much  greater 
and  the  chances  of  success  correspondingly  lessened.  Will  those  most 
vitally  interested  see  that  the  work  is  undertaken  ? 


